Good that you got it working.. I'm actually surprised that you said you replaced the zener diode ZD101 with a 1n4148, that's just wrong. The transformer winding marked B1 and B2 is the bias winding, it's a secondary winding really, that provides startup voltage and bias voltage for the MIP3E3SMY device, which is a 3 terminal flyback controller from Panasonic, licensed from Power Integrations, Inc. It's not simply a MOSFET, although it has a power MOSFET inside it. The B1 and B2 of the transformer is the bias winding, it is a secondary winding, and the voltage produced is half-wave rectified with a standard diode D102, and filtered with C110. Then the zener diode that you replaced with a standard 1N4148 is supposed to limit the rectified and filtered voltage to 12V. That diode ZD101 is a MA4120, a 12V zener. If you put a regular diode there, in the same orientation, it's just going to be reversed biased and do nothing at all, so of course it works, it's like the zener is not there anymore, and so the voltage may be too high now. If you carefully measure the voltage across the new diode you put in, you might find that it's above 12V. This may or may not hurt the MIP3E3SMY controller, I don't know. You would be wise to find a proper 12V zener and put it there, to keep it from future damage.
The original diode you had a question on is definitely a TVS device, as I said previously. It's drawn as a simple diode in the schematic, but it's really a diode and a zener as I explained. It's across the primary winding so that it can act as a freewheeling diode, for back EMF voltages above 170V, which may be generated when the MOSFET turns off.